Now that you've read the dummies guide to how the respiratory system works, it will be pretty easy to understand what happens when you smoke. There are more than 4,000 chemicals inside each and every cigarette [source: American Heart Association]. One of these is carbon monoxide (CO). Because it's tasteless and odorless, CO is often called the "silent killer." It's produced when fuel is burned -- gasoline, propane, natural gas, oil, wood or coal. This gas is deadly to humans when we're exposed to great deals of it in an enclosed area. People who commit suicide by locking themselves in the garage with a running car die from CO poisoning. Chances are, you have a carbon monoxide detector in your home -- it may be integrated into your smoke detector.

Burning a cigarette creates carbon monoxide that you suck into your lungs along with 3,999 other chemicals. When you smoke, the CO takes the place of the oxygen that we talked about in respiratory 101. This essentially poisons those red blood cells and prevents them from being able to carry oxygen to the heart, and from the heart to the rest of the body. How or why does this happen? It's easy -- CO is a master of disguise. It actually passes itself off as oxygen. In fact, it does such a good job at impersonating oxygen that your FedEx man, hemoglobin, is 200 times more likely to stop and pick it up for delivery than it would oxygen.

Since there's only so much room in your cells for these gases, something's got to go. Because you're forcing CO into your bloodstream with every puff, your body really has no choice -- the loser of this battle is oxygen. If you smoke a pack of cigarettes a day, the CO level in your bloodstream will stay between 4 and 8 percent [source: The New York Times]. A normal amount of CO in your blood is super low -- zero to eight parts per million. So now you have 7 to 15 percent less oxygen thanks to that pack of smokes. But your body still needs 100 percent to function properly. Seems like a bit of a quandary, doesn't it? The result is that your heart has to work harder and faster in order to distribute the required amount of oxygen to the rest of your body.

The short term effect of all this is that your heart rate increases and you feel short of breath. Carbon monoxide in the blood also leads to a buildup of fat on the walls of your arteries. A buildup of fat means that the hollow arteries that carry blood to your heart become more narrow and decrease the flow of blood. This is called atherosclerosis, and it's a leading cause of heart disease.

So is all lost? Are smokers permanently damaged? Are they doomed to die because they're basically sucking on a car tailpipe every time they smoke? Fortunately, the body is a resilient machine that does everything it can to stay alive. As soon as you've had your last cigarette, your body begins to repair itself and your CO levels drop pretty fast. Patients have shown that just one week away from their smoky treats will bring the level back to that of a non-smoker [source: Health Line]. This means an almost immediate improvement in your health once you put the pack down. Of course, once you pick up another cigarette, the process starts all over again. Think about this next time you start your car.

More Great Links

"Carbon Monoxide Exposure from Cigarettes: Just one more Reason to Quit." carbon-monoxide-poisoning.com. 2008. http://www.carbon-monoxide-poisoning.com/article3-carbon-monoxide- exposure-cigarettes.html